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icons-hash: use a better PRNG 

the older method was using a multiplicative congruential generator (MCG)
which doesn't work too well especially with just 32 bits of state.

change it to a PCG instead with 64 bits of state (and 32 bits of output)
which should give better results.

and since we should get better rng - the search iteration has been
halved as well to save some build time.
This commit is contained in:
NRK 2023-01-01 02:49:17 +06:00
parent 39aab7dac4
commit a47cc98ed6
1 changed files with 17 additions and 3 deletions
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20
src/icons-hash.c
View File

@ -9,6 +9,7 @@
#include <inttypes.h>
#define GOLDEN_RATIO_32 UINT32_C(2654442313) /* golden ratio for 32bits: (2^32) / 1.61803 */
#define GOLDEN_RATIO_64 UINT64_C(0x9E3793492EEDC3F7)
#define ICONS_TABLE_SIZE 8 /* size in bits. 8 = 256 */
#ifndef TOUPPER
@ -33,7 +34,7 @@
#define ASSERT(X) assert(X)
#define ARRLEN(X) (sizeof(X) / sizeof((X)[0]))
#define MAX(A, B) ((A) > (B) ? (A) : (B))
#define HGEN_ITERARATION (1ul << 14)
#define HGEN_ITERARATION (1ul << 13)
#define ICONS_PROBE_MAX_ALLOWED 6
#define ICONS_MATCH_MAX ((size_t)-1)
@ -100,6 +101,17 @@ table_populate(unsigned int p[static PROBE_CNT])
return p;
}
/* permuted congruential generator */
static uint32_t
pcg(uint64_t *state)
{
uint64_t oldstate = *state;
*state *= GOLDEN_RATIO_64;
uint32_t r = (oldstate >> 59);
uint32_t v = (oldstate ^ (oldstate >> 18)) >> 27;
return (v >> (32 - r)) | (v << r);
}
int
main(void)
{
@ -107,6 +119,7 @@ main(void)
assert(ICONS_TABLE_SIZE < 16);
assert(1u << ICONS_TABLE_SIZE == ARRLEN(table));
assert((GOLDEN_RATIO_32 & 1) == 1); /* must be odd */
assert((GOLDEN_RATIO_64 & 1) == 1); /* must be odd */
assert(hash_start > 1);
assert(hash_mul > 1);
/* ensure power of 2 hashtable size which allows compiler to optimize
@ -116,6 +129,7 @@ main(void)
unsigned int max_probe = (unsigned)-1;
uint32_t best_hash_start, best_hash_mul, best_total_probe = 9999;
uint64_t hash_start_rng = hash_start, hash_mul_rng = hash_mul;
for (size_t i = 0; i < HGEN_ITERARATION; ++i) {
unsigned *p = table_populate((unsigned [PROBE_CNT]){0});
@ -127,8 +141,8 @@ main(void)
best_hash_start = hash_start;
best_hash_mul = hash_mul;
}
hash_start *= GOLDEN_RATIO_32;
hash_mul *= GOLDEN_RATIO_32;
hash_start = pcg(&hash_start_rng);
hash_mul = pcg(&hash_mul_rng);
}
assert(max_probe < ICONS_PROBE_MAX_ALLOWED);
hash_start = best_hash_start;